Coordination bonds, dipole moments

Two types of parameters appear in this expression, m/ represents bond dipole moments, while dm/ / d/ /. represents derivatives of the bond dipole moment with respect to the internal coordinates (bond stretching, angle deformation, out-of-plane deformation, and torsion). These parameters are referred to as electro-optical parameters (eop). All other quantities are derived from the structure or from the normal coordinate calculation. The electro-optical parameters can be derived from measured intensities, like force constants are derived from measured frequencies. Compared to the determination of force constants, the problem in this case is that the number of parameters is much higher. 

The dipole moments of a variety of coordination compounds show that the bond dipole moments of the M-L bonds of most a-donor ligands are about 4 D, with the donor atom positive. In contrast, metal carbonyls show an M-C bond moment which is essentially zero because the M L back donation compensates for L M direct donation plus the enhanced polarization of CO on binding. Formation of the M-CO bond weakens the C-O bond relative to free CO because a n orbital on CO is now partly filled by back donation. This will still lead to a stable complex as long as the energy gained from the bond exceeds the loss in C-O. Bond weakening within L on binding to a metal is a common feature in many M L systems. 

Three types of intensity parameters oiter Eq. (6.59). These are die bond dipole moments Pk, die first derivatives of bond moments widi respect to internal coordinates, and second derivatives d pk/dR dR],. The latter quantities are termed electro-optical anharmonic parameters. These toms reflect the non-linear dependence of Pk on the vibrational coordinates Bj and are determined by die electrical anharmonicity of molecular vibrations. As in the charge flow formulation, harmonic terms miter the expressions for intensities of binary overtone and combination bands. 

The angles ot, p, and x relate to the orientation of the dipole nionient vectors. The geonieti y of interaction between two bonds is given in Fig. 4-16, where r is the distance between the centers of the bonds. It is noteworthy that only the bond moments need be read in for the calculation because all geometr ic features (angles, etc.) can be calculated from the atomic coordinates. A default value of 1.0 for dielectric constant of the medium would normally be expected for calculating str uctures of isolated molecules in a vacuum, but the actual default value has been increased 1.5 to account for some intramolecular dipole moment interaction. A dielectric constant other than the default value can be entered for calculations in which the presence of solvent molecules is assumed, but it is not a simple matter to know what the effective dipole moment of the solvent molecules actually is in the immediate vicinity of the solute molecule. It is probably wrong to assume that the effective dipole moment is the same as it is in the bulk pure solvent. The molecular dipole moment (File 4-3) is the vector sum of the individual dipole moments within the molecule. 

Furazano[3,4-/]quinoxaline, 7,8-diphenyl-synthesis, 6, 412 Furazanothiophene synthesis, 6, 417 Furazans, 6, 393-426 biological activity, 6, 425 bond angles, 6, 396 bond lengths, 6, 396 coordination compounds, 6, 403 diamagnetic susceptibilities, 6, 395 dipole moments, 6, 395, 400 heats of combustion, 6, 400 heterocyclic ring reactions, 6, 400-403 IR spectra, 6, 398 isoxazoles from, 6, 81 mass spectra, 6, 399 microwave spectroscopy, 6, 395, 396 MO calculations, 6, 395 monosubstituted... 

The obtained results are in agreement with our previous hypothesis [2-4] that absolute intensities and the distribution of relative intensities of IR bands in the spectra of adsorbed species are sensitive to the chemical activation of the corresponding bonds arising from polarization by adsorption sites. Hence, in addition to the low frequency shifts, intensites can be used as a criterion for chemical activation. Indeed, according to the fundamentals of IR spectroscopy, the intensities of IR stretching bands are proportional to the square of the dipole moment changes (dp) created by the stretching vibrations over the normal coordinates q of these vibrations [6] I °c [dp/d q]2. 

The qualitative example presented above describes the isolated formaldehyde molecule. In solution the large permanent dipole moment or hydrogen bonding of formaldehyde will induce an appreciable solvent electric field whose orientation in the molecular coordinate system is fixed (presumably parallel to... 

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